This invention relates in general to electrophotography, in particular, to charge generation layers and charge transport layers for electrophotographic imaging members, and to processes for preparing the same.
In electrophotography, an electrophotographic plate containing a photoconductive insulating layer on a conductive layer is imaged by first uniformly electrostatically charging its surface. The plate is then exposed to a pattern of activating electromagnetic radiation such as light. The radiation selectively dissipates the charge in the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image in the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic marking particles on the surface of the photoconductive insulating layer. The resulting visible image may then be transferred from the electrophotographic plate to a support such as paper. This imaging process may be repeated many times with reusable photoconductive insulating layers.
An electrophotographic imaging member may be provided in any of a number of forms. For example, the imaging member may be a homogeneous layer of a single material such as vitreous selenium or it may be a composite layer containing a photoconductor and another material. One type of composite imaging member comprises a layer of finely divided particles of a photoconductive insulating organic compound dispersed in an electrically insulating organic resin binder. U.S. Pat. No. 4,265,990 discloses a layered photoreceptor having separate photogenerating and charge transport layers. The photogenerating layer is capable of photogenerating holes and injecting the photogenerated holes into the charge transport layer.
More advanced photoconductive receptors contain more highly specialized component layers. For example, one type of multilayered photoreceptor that has been employed in electrophotographic imaging systems is schematically shown in FIG. 1, and comprises a substrate 11, a conductive ground plane 12, a charge blocking layer 13, a charge generation layer 14 (including photogenerating material in a binder), a charge transport layer 15 (including charge transport material in a binder), and an optional overcoating layer 16.
In photoreceptors of the type shown in FIG. 1, the photogenerating material generates electrons and holes when subjected to light. The blocking layer prevents the holes in the conductive ground plane from passing into the generator from which they would be conducted to the photoreceptor surface thus erasing any latent image formed there. The blocking layer, however, permits electrons generated in the generator to pass to the conductive ground plane, thus preventing an undesirably high electric field to build up across the generator upon cycling the photoreceptor.
In electrophotographic imaging systems such as the one schematically shown in FIG. 1, a particularly preferred charge blocking layer is formed of organo silicone compounds. These blocking layer compounds will hereafter be referred to as silane. The presence of a silane blocking layer has, however, been associated with several negative effects on the imaging process. First, it is believed that the silane layer tends to cause a print defect known as reticulation. Reticulation patterns appear in xerographic prints generally because of local areas of uneven thickness in the silane blocking layer, producing differences in the PIDC's in these areas. A PIDC is a measure of the amount of light that produces a given change in the surface voltage for a photoreceptor. Thus, the photoreceptor surface charge density is altered, resulting in a noticeable pattern in the print. These patterns are typically circular in nature. It is believed that reticulation is caused at least in part by a cellular pattern in the silane layer. Second, it is suspected that micro-white spots in printed images may appear when there are pin holes in such a silane blocking layer.
Selenium compositions are particularly preferred photogenerating materials, and such photogenerating materials are improved by doping with sodium. See U.S. Pat. No. 4,232,102 to Horgan et al. In particular, it is believed that sodium-doped selenium pigments provide improved cyclic stability, increased V.sub.bg, and increased charge acceptance. The background voltage, V.sub.bg, is the surface potential of the photoreceptor in the areas of the image that are derived from the white portion of the document being copied. High background voltage is undesirable because it indicates that the photoreceptor has lost sensitivity, i.e., it takes more light to generate a given surface voltage drop in forming the latent charge image. However, several defects have been attributed to the use of sodium-doped selenium photogenerating materials. For example, it is suspected that sodium doping of selenium results in an uneven coating of the selenium pigment. It is believed that such uneven coating causes an uneven discharge of the pigment, and may be a source of micro-white spots in printed images.
A second type of multi-layered photoreceptor comprising an inverted structure of several layers of the photoreceptor of FIG. 1 is schematically shown in FIG. 2, and comprises a substrate 21, a conductive ground plane 22, a charge transport layer 23, a charge generation layer 24, and a protective and blocking overcoating layer 25. Typically, layer 25 is an amorphous layer of 2% arsenic and 98% selenium. The top blocking layer is needed in this configuration to prevent holes from the corona charge from entering the charge generation layer and then discharging the negative charge which forms the image on the conductive ground plane.
The assignee of the present application has engaged in development of electrophotographic imaging systems such as the one shown in FIG. 2, and has recognized that such imaging systems occasionally exhibit low charge acceptance and a steep PIDC. A steep PIDC is one wherein a small amount of light produces a large change in surface voltage of a photoreceptor. PIDC sensitivity refers to the ability of a photoreceptor to produce a desired voltage drop with a small or large amount of light. A sensitive PIDC is one that indicates that the photoreceptor exhibits a given voltage drop with a small amount of light. Such systems also occasionally exhibit local areas of low charge acceptance, which lead to print defects described as gray spots. In some instances, the local discharge becomes great enough that the spots become large white areas. In some cases, such spots become worse with cycling.
There is a continuing interest in the development of photoreceptors of the above-described types in which manufacture is simplified, print defects are reduced, particularly over extended use, and useful life is lengthened.
U.S. Pat. No. 4,264,695 to Kozima et al. discloses an electrophotographic element which comprises an electrically conductive support, a first layer comprising a photoconductive substance capable of generating conductive charge carriers through light absorption and an electron donor or an electron acceptor, and a second layer comprising an electron donor or an electron acceptor.
U.S. Pat. No. 4,559,287 to McAneney et al. discloses a photoresponsive imaging member comprising a photogenerating layer and an electron transporting layer, wherein the charge transporting layer includes a stabilizing amount of an aryl amine electron donating compound. The stabilizing material prevents crystallization of the electron transporting layer.
U.S. Pat. No. 4,535,042 to Kitayama et al. discloses an electrophotographic photosensitive member with a layer comprising an electron acceptor and a layer comprising an electron donor, the layers being supported on a conductive substrate. The two layers are superposed upon each other to form a thin layer of charge-transfer complex at the interface between the two layers to utilize the thin layer as a charge generation layer.
U.S. Pat. No. 4,379,823 to Halm discloses a photoconductive coating layer comprising a combination of an organic photoconductive donor compound and an acceptorsensitizer compound. According to the patent, the coating layer may be used for coating a conductive substrate to provide a photoconductive film.
U.S. Pat. No. 4,337,305 to Beretta et al. discloses photoconductive layers formed by sensitizing organic electron donor compounds with dyes. The donor compounds may be combined with polymeric binder materials to form photoconductive layers which are charge transport layers.
U.S. Pat. No. 4,442,192 to Pai discloses a photoresponsive imaging member comprising a conductive substrate, a photoconductive layer containing a photoconductive material dispersed in a resinous binder, a hole trapping layer, and an overcoating layer comprising a composition capable of donating electrons to positive charges contained on the surface of the photoresponsive device.
U.S. Pat. No. 4,576,887 to Ehrlich et al. discloses photoconductive polymer compositions used for the preparation of photodetectors and photoconductive devices. According to the patent, the photoconductivity of the polymer may be enhanced by the addition of an electron-acceptor dopant. The patent notes that the conductivity of some electrically conductive or semiconductive materials can be enhanced through the use of electron acceptor and/or electron donor dopants.
U.S. Pat. No. 4,232,102 to Horgan et al discloses an imaging member comprising a layer of organic resin in which is dispersed a photoconductive material comprising trigonal selenium. This layer can be the charge generation layer in an imaging member also containing a charge transparent layer. The photoconductive material so prepared is useful for improving cyclic charge acceptance and control, and for improving dark decay.
U.S. Pat. No. 4,639,402 to Mishra et al discloses an imaging member comprising an organic resin binder and photoconductive materials containing selenium particles coated with a hydrolyzed aminosilane. This patent discloses only a coating process employing hydrolyzed aminosilane, and does not provide for adding the hydrolyzed aminosilane to the solvent of the coating solution.